In the Fischer-Tropsch reaction a gaseous mixture of carbon monoxide and hydrogen is reacted in the presence of a catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated hydrocarbons which typically have a chain length of more than 2 carbon atoms, for example, more than 5 carbon atoms.
It has recently been found that a Fischer-Tropsch process can be operated by dispersing a gaseous reactant stream comprising synthesis gas with a suspension of catalyst in a liquid medium in a continuous stirred reactor system. In such a continuous stirred reactor system, suspension is continuously introduced into a stirred reactor vessel and the rate of introduction of the suspension is balanced by the rate of withdrawal of suspension. Each increment of the suspension introduced into the stirred reactor vessel is mixed with the suspension already present in the reactor vessel and any variations in the composition of the suspension which may occur in the reactor vessel are averaged within time intervals shorter than the average residence time of the suspension within the reactor vessel resulting in the suspension being of substantially uniform composition. A gaseous reactant stream is continuously introduced into the reactor vessel while a gaseous purge stream is continuously removed either directly or indirectly from the reactor vessel. Any differences in the composition of the gaseous phase which is dispersed in the suspension are averaged within time intervals which are shorter than the average residence time of the gaseous phase within the suspension in the reactor vessel. Accordingly, the catalyst will be exposed to a uniform concentration of gaseous reactants. Mixing may be achieved within the reactor vessel by means of a mechanical agitator, for example a rotating impeller. Alternatively, mixing may be achieved by imparting turbulence to the suspension by passing the gaseous reactant stream and suspension through a high shear mixing zone, for example an injector mixing nozzle, wherein the gaseous stream is broken down into gas bubbles and/or irregularly shaped gas voids. The resulting mixture is then discharged into the reactor vessel where mixing is aided through the action of the gas bubbles and/or the irregularly shaped gas voids on the suspension. A Fischer-Tropsch process which employs such a turbulent continuous stirred reactor system is described in WO 0138269 (PCT patent application number GB 0004444).
Synthesis gas may contain high levels of carbon dioxide arising from the hydrocarbonaceous feedstock (for example, natural gas) employed in the synthesis gas production process or as a by-product of such a process. Many cobalt containing Fischer-Tropsch catalysts deactivate in the presence of even low concentrations of carbon dioxide. It may therefore be necessary to separate carbon dioxide from the synthesis gas before feeding the synthesis gas to a Fischer-Tropsch process. However, carbon dioxide may also arise as a by-product of the Fischer-Tropsch synthesis reaction. Where a Fischer-Tropsch process is operated using a fixed catalyst bed positioned in a plug flow tubular reactor, the concentration of carbon dioxide in the gas passing through the bed will increase with increasing distance along the bed. Consequently, the rate of deactivation of a susceptible catalyst will increase along the fixed bed. In contrast, in a continuous stirred reactor system, the catalyst will be exposed to a constant concentration of carbon dioxide. Consequently, a susceptible catalyst will decompose at a constant rate throughout the suspension. It is therefore critical that the catalyst used in a continuous stirred reactor system is stable to low amounts of carbon dioxide. It would also be advantageous to employ a catalyst which is stable in the presence of high amounts of carbon dioxide since this will avoid the need to separate carbon dioxide from the synthesis gas before the synthesis gas enters the continuous stirred reactor system.